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ISSN 1993-890X

The technology of tomorrow for general lighting applications. Nov/Dec 2008 | Issue 10

LpR

White Light LEDs

Importance of AcceptedMeasurement Standards


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LED professional Review | Nov/Dec 2008 | pagewww.led-professional.com 2

Copyright 2008 Luger Research & LED-professional. All rights reserved.

White Light LEDs Importance of AcceptedMeasurement Standards> Dr. Thomas Ngele, Instrument Systems GmbH

The breathtaking development of white light LEDs in recent years has led

to significant market potential for a range of brand new applications. The

high efficiency of these LEDs now means that solutions for room lighting,

automotive headlights and backlighting for LCD displays can be realized.

The advantages of their reduced energy requirements are apparent - for

example, the running time of battery operated devices such as mobile

phones and notebook computers can be extended considerably.

For general room lighting, the ability to vary the color temperature of a

LED lamp allows new lighting designs. In automotive applications, the

reduced energy requirement helps to reduce petrol consumption and

CO2 emissions. Moreover, their long lifetime minimizes service

requirements for headlights - i .e. the LEDs will in all probability outlive

the lifetime of the car itself. And lastly, the technical characteristics of

LEDs also bring significant advantages for adaptable headlight

technology when compared to conventional forms of lighting. All of

these aspects additionally mean that these developments in automotive

safety may also find application in a wider market.

Attaining and maintaining the necessary regulatory lighting standards

still represents challenges for LED manufacturers, and much research is

currently being made into methods to reduce and control the varying

quality inherent in the production process. Indeed, the proper

characterization of the light emission parameters forms an important

aspect of the quality control process. However, this characterization, in

particular for white light LEDs, is as a consequence of their optical

properties somewhat complicated.

Optical Properties of White Light LEDsThe intercomparability of measurement results is influenced by a variety of

factors. Both the fundamental technology underlying the structure of the

LED itself and the measurement system used for characterization can play

a decisive role. In order to ensure precise measurement of the LED emission,

spectroradiometers have now become the preferred tool of choice over

photometers and colorimeters. Spectroradiometers achieve better accuracy

by obviating the need for optical filters and by making a computational

calculation for the photometric and colorimetric parameters.

Inherent properties of the LEDs directly resulting from the manufacturing

process also influence the measurement results. White light LEDs,

usually comprising a blue emitting LED-chip coated with a yellow

emitting phosphor, are particularly difficult to manufacture with

consistent characteristics. Furthermore, well known factors such as

burn-in time, operating temperature, aging behavior, etc., each make

their own contribution to the emission performance of the LED.

Dependence on TemperatureAbove all for modern high-power LEDs, proper thermal management

and the operating temperature of the LED play a critical role in terms of

reproducibility of operation. The energy density in such devices is

considerably higher than that for conventional LEDs, resulting in the

generation of significantly more heat. Continuous operation of high-

power LEDs remains a major technical challenge, as varying temperature

can be responsible for changes in the emission spectrum.

Figure 1: Dependence of the spectral emission of white light LEDs on temperature

Figure 1 illustrates the spectral dependence of an LED emitting warm

white light over a temperature range from 20 to 80C. Generally, in

order to maintain a constant and uniform impression of brightness and

color in continuous operation, the heat generated in high-power LEDsmust be effectively dissipated. For characterization purposes, however,

unambiguous indication of the thermal behavior of an LED is best

gained through active cooling, for example through the use of a Peltier

element, thus allowing examination of the LED at predetermined

temperatures. As can be clearly seen in Figure 1, for increasing

temperature the blue emission peak shifts to longer wavelengths and

the total emission intensity decreases (the latter effect being typical of

LEDs in general). These spectral and intensity changes also cause an

alteration in the perceived color. In all industries associated with light

emission, perceived color has long been described by the concept of

color temperature. Table 1 below shows the calculated effective (i.e.nearest) color temperature for the LED emission illustrated in Figure 1.

Operating Temperature [C] Color Temperature [K]

80 3222

70 3203

60 3187

50 3173

40 3161

30 3151

20 3144

10 3141

Table 1: Change in the color temperature Tn for changing operating t emperature of a white light LED
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Between 20 and 80 C, the effective color temperature is shifted

considerably and leads to a significant change in the perceived color.

This property of white light LEDs is especially problematic in general

lighting applications and hinders the development of qualitative high-

grade products.

An additional issue is visible during the burn-in process for LEDs. That

is, as long as thermal equilibrium has not yet been reached, the electrical

and optical characteristics of the LED also undergo change.

Figure 2: The red squares show the dependence of the total irradiance of a white light LED after switching on, the

blue squares show the changing color temperature Tn

Figure 2 illustrates the burn-in process of a white light LED stabilized to

40C - the photometric luminous flux (black squares) of the LED falls

over a period of 10 seconds, while at the same time the effective color

temperature rises (white squares). This relatively long stabilization

phase must be accounted for if comparison of measurement data is to

be made, i.e. measurements made in the production environment can

differ from those made under stable laboratory conditions.

The final production step for LEDs is the optical characterization and

the subsequent sorting into so-called BINS. Typically, this measurement

and classification process occurs 20 to 30ms after the LED is switched

on, even if the LED has not yet been thermally stabilized. Nonetheless,

the specifications given by the manufacturer in the data sheet usually

correspond to this type of production environment measurement.

Customers receiving the LED must then often perform their own

measurement under stable conditions in order to appropriately

characterize the LED for the intended use.

Spatial Radiation PatternIn order to properly determine their color temperature, the spatial

radiation pattern of white light LEDs has to be considered. Contrary to

single color LEDs, the perceived color of a white light LED can change as

a function of the emission angle. This is a direct consequence of the

nature of the technology utilized by the manufacturer to coat the blue

LED-chips with phosphors. Differing standards among manufacturers

mean that this change can amount to more than 1000 K.

Using a combination of goniometer and spectroradiometer, the luminous

intensity distribution and the color temperature can be measured over

the spatial radiation pattern of a white light LED. Figure 3 depicts these

measurements for both a modern high-power white light LED and a

standard 5mm LED.

Figure 3: Spatial radiation pattern and color temperature of the emission of white light and standard LEDs, mea-

sured at a distance of 250mm

The luminous flux of high-power LEDs is usually measured with an

integrating sphere, alternatively the averaged luminous intensities

ILED-A or ILED-B. With an integrating sphere, the measured spectrum is

integrated over the entire emission hemisphere, whereas a simple

luminous intensity measurement only gathers light over a narrow solid

angle. If the color temperature is determined under conditions for

measuring ILED- B, the result is different to that obtained with an

integrating sphere (table 2) .

Color temperature

high-power LED [K]

Color temperature

5mm LED [K]

ILEDB 3145 5748

Integrating sphere 3112 6804

Table 2: The effective color temperatures for both LEDs as determined from measurement conditions appropriate

to that for an integrating sphere and for ILED-B

Were the discrepancy for the measured effective color temperatures of

a white light LED as apparent as that for the standard LED (table 2) , the

measurement geometry used to determine the specifications wouldhave to be indicated appropriately in the data sheet.

Aging of white light LEDsModern high-power LEDs require a relatively long burn-in phase until

little or no further change occurs over time. The aging process of a 1W

white light LED over a time period of 2000 hours is shown in Figure 4 [1].

Over this time frame not only the brightness varies, but also the spectral

distribution. During the aging process, the effective color temperature

changes by several 100K from 6900 to 6200K.


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Figure 4: Aging of a high-power white light LED

Lighting manufacturers have to take this behavior into account at the

design stage of the product in order to ensure that there are no quality

issues associated with degradation of performance. Intelligent feedback

and control of the LED over its entire lifetime is one solution to this

problem.

Measurement TechnologyAs highlighted above, spectroradiometers have now become the

standard for the measurement of the optical parameters of LEDs.

Compared to currently available photometers and colorimeters,

spectroradiometers obviate the need for the spectral filters that,

through tolerances in the manufacturing process, are often not

spectrally matched to the tristimulus curves. However, the optical

performance of the spectrometer is also a determining factor for the

validity of the measurement result. Two critical parameters are the

spectral resolution and the suppression of stray light.

Spectral ResolutionIn general, the spectral resolution of a spectrometer can be determined

by measuring laser light whose spectral linewidth is much smaller the

resulting full width half maximum of the spectrum then corresponds

to the spectral resolution of the spectrometer.

For broad-band continuous light sources such as halogen lamps, thespectral performance of a spectrometer has no recognizable influence

on the measurement. However, for narrow-band LED sources, insufficient

spectral resolution can artificially broaden the measured emission

spectrum and thus lead to falsification of the color values.

The emission from white light LEDs usually consists of a relatively

narrow band of blue emission superimposed on the spectrally broad

emission of the coating phosphor. For proper determination of the

effective color temperature, it is important to know the exact height

and width relationship for the blue component of the light relative to

that of the spectrally broader yellow phosphor. If the spectral resolution

of the spectrometer is too low, the blue range can be falsely represented.

Just as for a laser line, the measured blue spectrum of the LED is

broadened and the peak reduced. In contrast, the broader, yellowish

component of the phosphor emission is not affected. The net result

however is a change to the overall spectrum.

The effect that the spectrometer resolution can have on a measurement

is illustrated in table 3. The same LED was measured using a spectrometer

with variable resolution.

Spectral

resolution [nm]

Photometric Integral

[cd]

Color Temperature

[K]

1 0.877 5699

2 0.875 5695

5 0.872 5692

10 0.873 5676

Table 3: Dependence of the effective color temperature on the spectral resolution

While the photometric value remains more or less unchanged, the effective

color temperature shifts by 24 Kelvin. In order to guarantee sufficientaccuracy for the measured color coordinates, the spectral resolution of

the spectrometer should generally be in the range of 2 to 3nm.

Stray LightIn addition to the spectral resolution, the stray light performance of a

spectrometer also plays a critical role for the correct measurement of

color coordinates for white light LEDs.

The exact determination of the stray light behavior of a spectrometer

over the entire spectral range normally requires the use of a tunable

laser source in a complex measurement scheme [2] . However, with help

from a halogen lamp and a suitable 450nm long pass optical filter, the

stray light suppression afforded by a spectrometer for broad-band light

sources can be adequately assessed. Figure 5 shows measurement of

the filtered light from the halogen lamp for 3 different spectrometer

types - 2 array spectrometers and a double monochromator. The former

achieve 2.5 or 3.5 orders of magnitude for stray light suppression, the

double monochromator significantly more.

Figure 5: Stray light suppression for different spectrometers for filtered light from a halogen lamp: double mono-

chromator (black), array spectrometer #2 (blue), array spectrometer #1 (red)


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In the case of spectroradiometers, these are usually calibrated with the

use of halogen lamps with broad-band emission. A common standard,

available from almost all national laboratories, is a 1000W FEL lamp

with a color temperature of approx. 3100 K.

Halogen lamps have the disadvantage that the emission energy in the

blue spectral range only reaches roughly 10% of that at 800nm. The

total signal measured per wavelength interval during the calibration

process is the sum of the light from the halogen lamp in that same

interval plus the stray light component. Insufficient stray light

suppression in the blue can thus lead to erroneous values in this region

of the calibration file. For the array spectrometer #1, the stray light

contribution around 430nm amounts to almost 5%, while that for the

reference double monochromator is negligible due to its superior stray

light suppression.

Figure 6: Spectrum of a white light LED measured with three calibrated, but different spectrometers

The effect this can have on measured values for white light LEDs is seen

in Figure 6. The spectrum from the same LED as in Figure 5 is recorded

by the same 3 spectrometers using coupling optics corresponding to

those required for ILED-B measurements. For the array spectrometer

#2 (the one with the highest stray light contribution), the blue spectral

peak is noticeably smaller than for the other two spectrometers.

Normalization of the actual measurement values is made through use

of the calibration file, but the erroneous values in the blue spectral

region within this file lead to wrongly normalized data in this part of the

spectrum.

The color coordinates and the color temperature are affected directly

(see table 4) , while the photometric values are not.

SpectrometerPhotometric

Integral [cd]

x-

coordinate

y-

coordinateTn[K]

Double Monochromator 51.9 0.3169 0.3425 6209

Array Spectrometer #1 52.1 0.3173 0.3444 6186

Array spectrometer #2 51.8 0.3181 0.3453 6138

Table 4: Effect of the stray light suppression of the different spectrometers on the color values

SummaryAs the key light source for the future, white LEDs represent a major

challenge for optical measurement methods. Factors directly affecting

the light source itself, such as operating temperature, the burn-in phase,

aging behavior, etc., have to be analyzed and specified properly in order

to ensure good reproducibility of the measurements. Furthermore,

proper optical parameters for the measurement system also determine

whether characterization of LEDs can be made accurately and

reproducibly. Differing measurement geometries for the very same LED

can also lead to entirely different results. All of these factors have to be

accounted before proper internationally comparable quality standards

can be established.

References:

[1] Dr. Adrian Mahlkow; O.U.T e.V.; private communication; May 2008

[2] Yuqin Zong et al; NIST; Simple spectral stray light correction method for array spectrometers, Applied Optics,

Vol 45, No 6; Feb. 2008

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